Considerable quantities of waste water are produced from pulp production, and environmental regulations stipulate that it has to be treated and suspended matter must be removed before it is led into the water system. The waste water is usually first led to treatment where most of the suspended matter therein separates and settles as sludge to the bottom of a settling pond. Hereafter the water is led to biological purification whereby suspended matter is also obtained. Both of these formed suspended matters are recovered and dried either by a press or in some other known manner. The formed sludge is led into a boiler, which typically is e.g. a bark fired boiler or the like, operating as the power boiler of the pulp mill. The chlorine content of the suspended matter obtained from the waste water is significant, the chlorine originating either from the bleach plant or from the wood used. The sulfur content in normal power boiler fuel, such as bark obtained from the debarking plant, is very low. As a result, a situation arises in the power boiler wherein the chlorine in the sludge reacts with the alkaline material entering the boiler, such as potassium, sodium and calcium, forming alkali chlorides which cause corrosion in the boiler heat transfer surfaces. This problem occurs especially in bubbling fluidized bed boilers, wherein the formed alkali chlorides are driven together with fly ash and flue gas onto the heat transfer surfaces of the boiler. As the alkali chlorides lower the melting temperatures of the ash sediments on the boiler heat transfer surfaces, they cause and enhance molten phase corrosion in their materials. In the long run this results in the formation of leakage in the boiler tubes, causing shutdown.
Materials that are very resistant to corrosion phenomena or materials causing corrosion are typically used in attempts to stop corrosion. Materials that are resistant to chlorine-induced corrosion have been used in power boilers. To use these materials is, however, quite costly, and superior materials do not eliminate corrosion, but only slow its progress owing to their superior resistance to corrosion. Consequently, materials with superior corrosion resistance have not in practice proved sufficiently effective in preventing chlorine-induced corrosion.
U.S. Pat. No. 5,124,135 discloses a process for removing elementary chlorine from gaseous mixtures of Cl.sub.2 and Br.sub.2. In this process gaseous sulfur dioxide is added to a gas mixture containing chlorine and steam, and then the mixture is cooled until a purified gas is obtained. The publication states that as a result also water drops are obtained, but the mechanism of the chlorine removal reaction is not disclosed. The significance and role of sulfur dioxide in the reaction also remains unclear.
The Finnish Patent Application 933,336 discloses a method of adjusting the sulfur balance in a sulphate pulping process, wherein chlorides in the chemical recovery loop are removed in a recovery boiler by feeding sulfurous odor gases as such into the boiler. This aims at invoking a reaction between sulfur and chlorine to separate the chlorine in gaseous form. Adjusting the chemical recovery loop in a pulping process and the thereto related recovery boiler burning are as such a fully different kind of technique than the present invention applies to. Furthermore, the solutions in this application cannot as such be applied to a power boiler using biofuel, since the processes and conditions therein are substantially different.